Private Autonomous System (AS) Removal Requirements
draft-mitchell-grow-remove-private-as-00

GROW                                                         J. Mitchell
Internet-Draft                                     Microsoft Corporation
Intended status: Informational                             July 15, 2013
Expires: January 16, 2014

          Private Autonomous System (AS) Removal Requirements
                draft-mitchell-grow-remove-private-as-00

Abstract

   This document specifies operator's requirements for implementations
   that remove Private Use Autonomous System (AS) numbers from the AS
   path of routes sent to external Border Gateway Protocol (BGP) peers.

Status of This Memo

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1.  Introduction

   After the original IANA reservation of Autonomous System Numbers
   (ASNs) for Private Use was allocated via [RFC1930] implementation
   specific features were released that removed Autonomous System
   Numbers (ASNs) from the Border Gateway Protocol AS_PATH attribute.
   The details of such implementations were driven by multiple operators
   use cases and varied accordingly.  At times, implementation
   differences and undocumented behaviors have led to operators leaking
   Private Use ASNs to the Internet.  Now that a new range of Private
   Use ASNs has become available via
   [I-D.ietf-idr-as-private-reservation] implementations will likely
   require update and even more variation is possible.

   This document captures operator's requirements across various use
   cases, being cognizant of the operations of current implementations
   that remove Private Use ASNs, and provides a set of requirements for
   Private Use ASN removal implementations in the hopes of reducing
   inconsistencies and variations between implementations.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Basic Requirements

   An implementation that removes Private Use ASNs MUST provide a
   configuration option to remove them from both the AS_PATH attribute
   of [RFC4271] and if Four-Octet AS Support [RFC6793] is present, the
   AS4_PATH attribute of the route.  This configuration option MUST be
   configurable at the External Border Gateway Protocol (EBGP) peering
   session level, i.e. per neighbor, and will impact the as path
   attributes associated with any NLRI sent to the router to which is
   configured.  The implementation MUST remove all Private Use ASNs from
   the as path attributes up to the first non-Private Use AS in the as
   path, except as dictated by Section 4.  An implementation MAY remove
   Private Use ASNs from the entire as path (past the first ASN in the
   as path attributes), however if it does so, it SHOULD provide an
   operator configurable option to disable this behavior if desired.
   The reason for this behavior is that operators would prefer
   visibility to which network is leaking Private Use ASNs to the global
   Internet (or any other network) so the behavior can be corrected
   directly by the upstream network providing connectivity to the
   Private Use ASN rather than masking the issue, which may not fully
   correct the problem if the upstream network has multiple providers.

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4.  Loop Prevention when using Private Use ASN Removal

   Implementations of the Private Use ASN removal feature SHOULD provide
   basic loop prevention to prevent a multi-homed network with a Private
   Use ASN from accepting a prefix that was originated by its other
   location when the route is passed back to it from an adjacent ASN who
   may have the "Remove Private AS" functionality enabled, and has
   removed the network's Private AS from the path.  Due to the standard
   BGP path selection process described in Section 9.1.2.2 of [RFC4271]
   EBGP routes will be preferred over IBGP routes which may have been
   from IBGP neighbors within the AS, so without further attribute
   manipulation, this can pose a risk of a routing information loop to
   some networks.  Therefore a router SHOULD NOT remove Private Use
   ASN's from an AS_PATH or AS4_PATH attribute if it encounters the EBGP
   AS of the neighbor on which it is configured in the AS_PATH or
   AS4_PATH that would be removed.

5.  Unnecessary Restrictions on Local or Peer AS

   Implementations of this feature SHOULD NOT have any unnecessary
   restrictions on Private Use ASN use on either the local ASN of the
   router that is configuring the feature or the peer ASN that will be
   receiving the routes.  Both use cases are prevalent in some networks
   as Private Use ASN removal features have sometimes been used in
   network mergers or other situations where masking the Private Use
   ASN's behind a particular AS is necessary to avoid conflict with
   Private Use ASN's behind the upstream network.  In these cases, as
   long as both the router configuring the feature and the peer have a
   unique Private ASN from each other, all routes originated from behind
   their networks containing Private ASN's can be masked to be their
   ASN.  In the case where the AS where the feature is configured is a
   Private Use ASN and the router also has policy configured to prepend
   the local AS to the as path, an implementation SHOULD NOT remove the
   ASN's that have been locally prepended as per policy configuration.

6.  Behavior Towards other Special-Use ASNs

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   Implementations of this feature SHOULD NOT remove Documentation ASNs
   [RFC5398] as this may encourage their use by operators.  These ASNs
   are not reserved for Private Use and use of them is likely the result
   of a misconfiguration.  Due to historical reasons and lack of
   operator guidance on Last ASNs prior to
   [I-D.jhjm-idr-last-as-reservations] implementations MAY remove Last
   ASNs, which are deployed in some networks as if they are Private Use
   ASNs, even though this is not recommended to operators for the
   reasons specified in that document.  If implementations choose to do
   this, the behavior towards Last ASNs SHOULD be consistent with the
   behavior of the implementation towards Private Use ASNs as specified
   in this document.

7.  Operational Considerations

   It should be noted that removing items from the AS_PATH or AS4_PATH
   poses some risk and could introduce the chance of a routing loop.
   Further operational considerations for the use of Private Use ASNs
   are documented in [I-D.ietf-idr-as-private-reservation].

8.  IANA Considerations

   There are no IANA actions required by this document.

9.  Security Considerations

   There are no new security concerns in relation to the feature
   described in this document.  General BGP security considerations are
   discussed in [RFC4271] and [RFC4272].  Identification of the
   originator of a route with a Private Use ASN in the AS path would
   have to be done by tracking the route back to the neighboring
   globally unique AS in the path or by inspecting other attributes.

10.  References

10.1.  Normative References

   [I-D.ietf-idr-as-private-reservation]
              Mitchell, J., "Autonomous System (AS) Reservation for
              Private Use", draft-ietf-idr-as-private-reservation-05
              (work in progress), May 2013.

   [I-D.jhjm-idr-last-as-reservations]
              Haas, J. and J. Mitchell, "Last Autonomous System (AS)
              Reservations", draft-jhjm-idr-last-as-reservations-00
              (work in progress), May 2013.

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC5398]  Huston, G., "Autonomous System (AS) Number Reservation for
              Documentation Use", RFC 5398, December 2008.

   [RFC6793]  Vohra, Q. and E. Chen, "BGP Support for Four-Octet
              Autonomous System (AS) Number Space", RFC 6793, December
              2012.

10.2.  Informative References

   [IANA.AS]  IANA, ., "Autonomous System (AS) Numbers", July 2013,
              <http://www.iana.org/assignments/as-numbers/>.

   [RFC1930]  Hawkinson, J. and T. Bates, "Guidelines for creation,
              selection, and registration of an Autonomous System (AS)",
              BCP 6, RFC 1930, March 1996.

   [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis", RFC
              4272, January 2006.

Appendix A.  Acknowledgements

   JM - Placeholder.

Author's Address

   Jon Mitchell
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   USA

   Email: Jon.Mitchell@microsoft.com

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